1. Field of the Invention
The present invention relates to an exposure timing determining method for determining a timing at which the exposure is to be started for a photosensitive member such as a photosensitive drum, and an image-forming apparatus to which this method is applied.
2. Description of the Related Art
Conventionally, an image-forming apparatus is known, wherein a printing paper sheet (recording medium), which is supplied from any one of various trays, is transported by a transporting belt, while a developing agent such as a toner is electrostatically attracted to an electrostatic latent image formed on a photosensitive drum by performing the exposure, for example, with a laser beam so that a toner image on the photosensitive drum, which is formed by the exposure and the development as described above, is transferred to the recording medium disposed on the transporting belt, and thus an image is formed on the recording medium.
Usually, such an image-forming apparatus comprises resist rollers which feed the printing paper sheet subjected to the paper feeding in a direction in which the photosensitive drum is positioned, and a position sensor which detects the arrival of the printing paper sheet by means of a movable segment pushed out thereby, the movable segment having been arranged to interrupt the route for the printing paper sheet. The image-forming apparatus is constructed such that when the printing paper sheet, which has passed along the resist rollers, is detected by the position sensor, the exposure is started at a predetermined timing after the detection. A guide member, which guides the printing paper sheet fed from the resist rollers in the direction in which the photosensitive drum is positioned, is arranged together with the position sensor in the space (hereinafter referred to as “transport space”) disposed between the resist rollers and the photosensitive drum.
In the case of such an image-forming apparatus, the difference arises in relation to the way or degree of warpage in the transport space, because the rigidity of the printing paper sheet differs due to the difference in the thickness of the printing paper sheet to be fed from any one of the various trays. As a result, the time (photosensitive member arrival time), which ranges from the passage of the printing paper sheet along the position sensor to the arrival at the photosensitive drum, is changed for every thickness of the printing paper sheet. The transfer position on the printing paper sheet is consequently deviated resulting from the difference in the photosensitive member arrival time.
In view of the above, an exposure timing-determining method is known, wherein the timing (hereinafter referred to as “exposure timing”), at which the exposure is to be started, is determined depending on the thickness of the printing paper sheet as the transfer objective, and thus the error, which relates to the arrival distance ranging from the sensor position for the printing paper sheet to the photosensitive drum (as well as the photosensitive member arrival time), is absorbed to adjust the transfer position on the printing paper sheet (refer to Japanese Patent Application Laid-open No. 2001-42743, for example).
However, in the case of the image-forming apparatus to which the conventional exposure timing-determining method is applied, even when the thickness of the printing paper sheet is identical, the deviation of the transfer position on the printing paper sheet sometimes exceeds an allowable range, for example, due to any other factor such as the transport route of the printing paper sheet. A problem arises in that such a situation is not considered.